Proximity sensor for portable wireless device

ABSTRACT

A proximity sensor for a portable wireless connected device, the sensor being arranged to determine whether a part of a user&#39;s body is near the portable connected wireless device, The sensor generates a time-averaged proximity that is asserted when the device is brought near a part of a user&#39;s body for a given time and may be periodically reset momentarily during the periods of proximity. An integration time comparable with that used in SAR testing, such that the sensor may be used advantageously to reduce the radio power emitted by a portable device when it is near the body, can be obtained by a sigma/delta modulator configured as rate-compression unit.

REFERENCE DATA

The present application claims the benefit of prior date of U.S.provisional patent application 63/297,089 of Jan. 6, 2022, the contentswhereof are hereby incorporated in their entirety.

TECHNICAL DOMAIN

The present invention concerns a smart proximity sensor and a circuitfor processing the output of a proximity sensor. The invention isconcerned especially, but not exclusively, with a connected portabledevice, such as a mobile phone or a tablet that is equipped with such aproximity sensor and processor and is arranged to adapt the RF emittedfrom a radio interface to maintain a Specific Absorption Rate (SAR),Power Density (PD), or any RF exposure within given limits.

RELATED ART

Capacitive proximity detectors are used in many modern portable devices,including mobile phones and tablets, to determine whether the device isclose to a body part of a user. This information is important in severalways: it is used to detect whether the telephone is being activelymanipulated by a user, and whether the user is looking at the display,in which case the information displayed can be adapted, and/or thedevice switch from a low power state to an active one. Importantly, thisinformation is used to adapt the power level of the radio transmitter tocomply with statutory SAR limits. Capacitive proximity detection is usedalso in touch-sensitive displays and panels.

Known capacitive sensing systems measure the capacitance of an electrodeand, when the device is placed in proximity of the human body (forexample the hand, the head, or the lap) detect an increase incapacitance. The variations in the sensor's capacitance are relativelymodest, and often amount to some percent of the “background” capacitanceseen by the sensor when no conductive body is in the proximity. Knowncapacitive detection systems may include a digital processor forsubtracting drift and noise contributions and deliver a digital value ofthe net user's capacitance in real time and/or a digital binary flagindicating the proximity status based on a programmable threshold.

Proximity sensors are used in portable wireless devices to reduce thepower of a radio transmitter when the device is close to the user'sbody, for example when a mobile phone is moved to the ear for making acall or put in a pocket. By reducing the power only when the device isclose, regulatory exposure limits can be respected, without compromisingthe connectivity excessively, since the device can transmit at maximumpower when it is not close to the body. EP 3402074 A1 and US 2015/237183A1 disclose such uses.

Exposure limits to radio energy are set by several national andinternational standards. They generally include both spatial (mass,surface) and time averaging conditions. The ICNIRP standard (74, HealthPhysics 494 (1998)) provides for averaging over 6 minutes at 10 GHz andreduces to 10 seconds at 300 GHz on a complex basis. The IEEE standard(IEEE Std C95.1-2019 (2019)) has an averaging time of 25 minutes at 6GHz dropping to 10 seconds at 300 GHz. The FCC(https://docs.fcc.gov/public/attachments/FCC-19-126A1.pdf) proposes anaveraging time of 100 seconds below 2.9 GHz dropping to 1 second above95 GHz.

It is known to limit the power of a radio transmitter in a portabledevice to keep the average SAR/PD value in a sliding time window belowthe regulatory safety limit. In this approach, the actual transmissionpower is reduced according to the monitored traffic, irrespective ofwhether the device is close to the user or not. These devices do notrely on a proximity sensor to respect the regulatory SAR/PD limits.

Proximity sensors are usually configured to generate a prompt proximitysignal, that is a signal that is immediately asserted as soon as thesensor is moved near to a target object. EP 3869691 A1 discloses asensor configured to output a time-averaged proximity signal.

SHORT DISCLOSURE OF THE INVENTION

An aim of the present invention is the provision of a device/method thatovercomes the shortcomings and limitations of the state of the art.These aims are attained by the object of the attached claims.

The present invention proposes a proximity sensor that, when it is usedin a portable wireless device, enables ways of reducing the SAR dose toa user of a portable device without compromising the connectivity toomuch.

In contrast with the known (immediate) approach in which the RF power isreduced as soon as the portable device is near the user, the proximitysensor is configured to generate a time/averaged proximity signal thatis not immediately asserted when the portable device is moved near theuser, but only after a certain time, if the proximity perdures.Importantly, the time-averaged proximity signal may be reset to zerobriefly even while the portable device is near the user, and thenreasserted again. In this way, the radio power is caused to risemomentarily. While this short increases in power do not add much to theintegrated SAR dose, they can improve considerably the data connection.Connectivity is less degraded and may not be reduced at all if theproximity is only transitory.

In a variant (immediate/time averaged), the proximity signal ensues assoon as the device is brought near the user, but the reduction of poweris not permanent. Rather, RF power is cycled between a high value and alow value such that SAR is reduced, but connectivity is less affected.

The two approaches can be combined, and a time averaged proximity signalmay be used to determine a cyclic reduction of RF power, rather than apermanent one.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are disclosed in the description andillustrated by the drawings in which:

FIG. 1 illustrates schematically a capacitive proximity sensor in aportable connected wireless device.

FIG. 2 illustrates the behaviour of a part of the processor of theinvention.

FIG. 3 shows schematically the same behaviour, as a flowchart.

FIGS. 4 and 5 plot the dose over distance and the power over time from amobile device using an immediate proximity flag and a time-averagedflag.

FIGS. 6 and 9 are schematic representations of a variants of theinventive processor.

FIGS. 7 and 8 plot the proximity signals generated by the inventivecircuit.

FIG. 10 is a schematic representation of a sigma-delta converter

FIG. 11 is a schematic representation of a sigma-delta converter as usedin variants of the invention

FIG. 12 is a schematic diagram of an embodiment

FIG. 13 is an improved embodiment

FIG. 14 shows an inventive method as a flowchart

FIG. 15 shows the action of the proximity sensor when a part of the bodyapproaches a mobile device, remains in contact for a while, thenretracts away

EXAMPLES OF EMBODIMENTS OF THE PRESENT INVENTION

FIG. 1 shows schematically a capacitive proximity detector in aconnected portable device such as a portable phone, laptop computer, ortablet, but the filter and the method of the invention could be appliedto diverse fields.

The detector is sensitive to the capacitance Cx of an electrode 20 thatwill increase slightly at the approach of a user's hand, face or body.The variations due to body proximity are overshadowed by the owncapacitance of the electrode 20 which, in turn, is not stable. Thecapacitance signal is preferably amplified and processed by an analogueprocessor 23, which may also subtract a programmable offset, andconverted into raw digital values by an A/D converter 25. The samplesR(n) may be encoded as 16 bits integers or in any other suitable format.

The raw samples R(n) contain also, in a non-ideal world, noise andunwanted disturbances that are attenuated by a filter 30, providing aseries of samples U(n) useful for the processing in the successivestages.

Preferably, the detector includes the drift-correction circuitrepresented here by elements 60 and 40.160 is a baseline estimator thatgenerates a series of samples A(n) that approximate the instantaneousvalue of the baseline, considering drift. This is then subtracted fromthe U(n) samples in difference unit 40 and provides the drift-correctedsamples D(n). A discriminator unit 50 then generates a binary value‘PROXSTAT’ that indicates the proximity of the user's hand, face, orbody. In the following, the ‘PROXSTAT’ variable is treated as a binaryvalue. The invention is not so limited, however, and encompassesdetectors that generate multi-bit proximity values as well. Thereference input 70 of the discriminator is a suitable threshold value,which may be predetermined at manufacturing, defined in an individual ortype calibration, set dynamically by an host processor, or defined inany other way.

Should the capacitive proximity sensor be part of a connected portabledevice for SAR control, the sensor electrode 20 will preferably beplaced close to the transmitting antenna of the RF transmitter, todetermine accurately the distance from the radio source. The sensorelectrode 20 could be realized by a conductor on a printed circuit boardor on a flexible circuit board and may have guard electrodes on the backand at the sides, to suppress detection of bodies and objects at theback or on the sides of the device.

In the same application, the capacitive electrode 20 could serve also asRF antenna, or part thereof. FIG. 1 shows this feature of the invention.The electrode 20 is connected, through a decoupling capacitor C_(d), toa radio transmitter and receiver unit 90, and has an inductor L_(d), oranother RF-blocking element, to block the radiofrequency signal.Otherwise, the radio unit 90 could be connected to an antenna separateand independent from the sense electrode 20 which, in this case, couldbe connected directly to the analogue interface 23 without thedecoupling inductor L_(d).

FIG. 2 show schematically a processor that processes the PROXSTAT signal310 to determine a time-averaged TIMEAVGSTAT status flag that is highwhen the user has been close to the device for some time. When the userapproaches the telephone to the body, TIMEAVGSTAT does not become highimmediately, therefore fleeting approaches do not cause a reduction intransmission power. When the telephone remains in closeness to theuser's body for some time, the TIMEAVGSTAT status flag is raised.

To function, the circuit of FIG. 2 has some form of memory that retainsa trace of past states of the immediate proximity flag PROXSTAT. Whileseveral variants are possible, this example has an accumulator 280 and aFIFO buffer 250. The PROXSTAT variable is available at terminal 310. theaccumulator 280 adds together the values of PROXSTAT each time a newvalue is available and is periodically reset to zero. The time betweensuccessive resets is predetermined and defines a granularity interval.

At the end of the granularity interval, before the resetting of theaccumulator 280, a new value is pushed in the FIFO buffer 250 by theserial input 370. If the value of accumulator 280 is zero, or below adetermined threshold, then a value ‘0’ is pushed in the FIFO. Otherwise,a value ‘1’ is pushed in the FIFO.

Preferably, the length of the FIFO buffer 250 is variable and can be setat will, within predefined limits. In an exemplary implementation thebuffer 250 can have a length of up to 256 places. The length of the FIFObuffer 250 and the granularity interval between each reset of theaccumulator 28 define the length of a sliding time window that is usedto average the immediate proximity status flag, relative to the rate ofgeneration of new PROXSTAT values.

Note that the purpose of accumulator 280 is to slow down the insertionof new values in the FIFO buffer and, consequently, to limit the lengthof the FIFO buffer 250 needed to obtain a given time window. The windowsize is determined in relation to the integration level allowed in theregulation and, if the desired window size were quite short and memorynot a limiting factor, the accumulator 280 could be dispensed with.

Note also that the present disclosure deals with the special case inwhich the immediate status flag PROXSTAT is a one-bit value, and thecontent of the accumulator 280 is quantized to one bit before beingpushed in the FIFO buffer. The FIFO buffer has therefore a width of onebit. This is not a necessary limitation, however, and the invention alsoincludes variants in which the immediate flag PROXSTAT is a multi-bitvariable, the accumulator 280 accumulates a suitable function ofPROXSTAT that indicates whether the device is near, and the valuespushed in the FIFO buffer 250 are also multi-bit variables.

Note also that the FIFO buffer 250 can be implemented in various wayswithout leaving the scope of the invention, for example with a shiftregister or a ring buffer.

The values comprised in the FIFO buffer 250 are samples of the immediatestatus flag PROXSTAT in a sliding time window, whose length is definedby the length of the buffer times the granularity interval betweensuccessive introductions of new values in the buffer. The adding unit220 sums all the values in the FIFO buffer—which, the values beingsingle bits, is the same as counting them—and the result is comparedwith a predetermined threshold 320 in the comparator 260 to produce atime-averaged proximity status flag 330. Preferably, the comparator 260has a hysteresis to avoid multiple transitions when the input value 360lingers close to the threshold value 320.

While the figure shows an adder 220 reading all the values in the FIFObuffer through the respective parallel outputs at each cycle, this isnot the only manner of implementing a sliding sum. A possible variant,for example, may include a register to which the new values entering thebuffer at one side are added, and the old values dropping out of theother side of the buffer are subtracted at each cycle. The block 259comprising the FIFO buffer 25 a and the adder 220 can be regardedfunctionally as an averaging, or as a sliding sum unit. Although therepresented variant is preferred, being stable and easy to implement,all possible implementations of averaging units or sliding sum units maybe adopted instead.

The time-averaged proximity status TIMEAVGSTAT could be used to modifythe power of a radio transmitter of a portable device, in lieu of theimmediate proximity status PROXSTAT. In a preferred variant, a logicunit 270 is used to generate a combined status PROXTIMESTAT, availableat terminal 350, that is the result of a logic operation on PROXSTAT andTIMEAVGSTAT. The logic operation may be a logic ‘or’, or a logic ‘and’,and is preferably selectable by a suitable variable PROXTIMECONFIG,corresponding to wire 340 in FIG. 2 .

FIG. 3 illustrates the behaviour of the invention in a flowchart. Themethod starts with the generation of a new value of PROXSTAT (step 105)that the circuit of FIG. 1 produces at periodic regular intervals. Instep 120, the accumulator 280, here indicated by the variable‘TimeGranCount’ is updated. In step 122 the system checks whether thecurrent granularity interval is complete. In most cases, the granularityinterval will not be complete, and the system will take the ‘N’ branch,update the value of the combined status PROXTIMESTAT (step 170) and endthe processing, until the next PROSTAT value is available.

At the end of a granularity interval, the invention pushes a new valuein the FIFO buffer (step 130) which new value may be a ‘0’ or a ‘1’ asdisclosed above, or another suitable value, if the FIFO buffer allowsmulti-bit values, the sliding sum TIMEAVGCOUNT is recalculated, comparedwith the threshold value TIMEAVGTHRESH (step 140) and the time-averagedflag TIMEAVGSTAT is set accordingly (steps 150 and 160).

Plots 4 and 5 illustrate how the power of a radio transmitter can becontrolled to respect SAR/PD limitations, in the invention. Plots 4 showthe situation in which the radio power is governed by the immediate flagPROXSTAT only. The left-side plot shows the dose level as function ofthe distance for two power levels: P2 is the full power, and P1 is areduced “safe” power that is selected by the immediate proximity statusPROXSTAT, trimmed to fire when the distance reaches the value D1 atwhich the dose at nominal power reaches the maximum admissible level‘L’. The right-side plot shows that the power level is ‘P2’ whenPROXSTAT (trace 310) is inactive and is immediately lowered to ‘P1’ whenPROXSTAT is active.

Plot 5 shows a case in which the output power is governed by thecombined status PROXTIMESTAT, computed in this case by a logic ‘and’ ofPROXSTAT (trace 310) and TIMEAVGSTAT (trace 330).

FIG. 6 shows a variant of the invention comprising a logic AND gate 271at the input of the averaging unit 259. The averaging unit isrepresented functionally as a block and its internal structure mayinclude the FIFO buffer 250 and sum unit 220 of FIG. 2 or have adifferent structure. The averaging unit 259 yields a value TIMEAVGCOUNT360 that count the accumulated length of time during which the proximitysignal PROXSTAT was active, in a time window of predetermined length. Ifthe averaging unit is implemented as disclosed in FIG. 2 , the windowlength will correspond to the depth of the FIFO times the update rate,which is determined by the rate of production of new PROXSTAT samples,scaled by the integration time of the counter 280, if present.

The value TIMEAVGCOUNT is compared with a suitable thresholdTIMEAVGTHRESH 320 in comparator 260, as in the previous embodiment. Atime-averaged proximity flag PROXTIMESTAT 350 is generated if thethreshold TIMEAVGTHRESH is exceeded and the PROXSTAT is active, asrepresented by the logic gate 273, which substitutes, in thisembodiment, the multiplexer 270 of FIG. 2 .

Importantly, the signal PROXTIMESTAT 350 is fed back to the input of theaveraging unit through the logic AND gate 271 that has its inputs tiedto the PROXSTAT value and to the complement of PROXTIMESTAT. In thisembodiment, the logic gate 271 inhibits the accumulation of new PROXSTATvalues if the time-averaged proximity signal PROXTIMESTAT is alreadyactive. This is advantageous when the sensor is used to limit the radiopower of a mobile device, since it allows the power to return to a highlevel in short intervals during the whole detection period, rather thanallowing a short time of high power only at the beginning, as in theprevious embodiment. The inventors have found that this manner ofdetecting proximity improves significatively the connectivity when thedetection period (the window length mentioned above) is rather long,i.e., spans over several minutes.

If, to make an example, the embodiment of FIG. 2 would yield a 2 minhigh power period at the beginning, and then low power for the rest ofdetection time, until the device is moved away, this improvedembodiment, thanks to the negative feedback disclosed above, would givewith similar parameters, a series of 2 min-periods of high poweralternated with periods of low power. In this manner, connectivity canbe preserved without worsening excessively the SAR.

Manufacturers also have the flexibility to use a shorter FIFO durationwhile still complying with the SAR limit computed on a longer regulatorywindow.

FIG. 7 shows the values of the immediate proximity status PROXSTAT (plot310), the corresponding values of “1” values present in the FIFO bufferTIMEAVGCOUNT (plot 360), the threshold TIMEAVGTHRESH (plot 320) and thetime-averaged proximity status TIMEAVGSTAT (plot 350). The digitalsignals 310 and 350 have been shifted by an arbitrary amount to improvereadability.

FIG. 7 corresponds to a scenario in which the mobile device istemporarily moved close to the user in four short intervals, as shown bythe immediate proximity status (plot 310). As explained above, thisleads to a rise of the TIMEAVGCOUNT value, without however reaching thethreshold level (line 320). Consequently, the time-averaged proximitystatus (plot 350) remains inactive all the time.

FIG. 8 corresponds to a situation where the proximity between the mobiledevice and the user is protracted, and the proximity sensor isconfigurated as in FIG. 6 . The accumulated value TIMEAVGCOUNT (plot360) rises steadily until the threshold value 320 is exceeded, whereuponthe time-averaged proximity status (plot 350) becomes active. The gate271 now inhibits the accumulation of further “1” values in the averagingunit 259, the accumulated value 360 after a constant period at highvalue dips below the threshold line 320, and the time-averaged proximitystatus becomes temporarily inactive, despite the continuing proximity.The cycle then repeats until the proximity ends.

FIG. 9 shows another variant in which the PROXSTAT signal is gated bylogic gate 271 and by a second OR gate 272 with an input receiving thecomplementary value of PROXSTAT. This variant works as that of FIG. 6when PROXSTAT is active. When PROXSTAT is inactive, however, theaveraging unit 259 is pre-filled with “1” values, which may provide afaster response. The logic gates at 271 and 272 simulate an activePROXSTAT value even though PROXSTAT is inactive and fill the memory ofthe averaging unit accordingly. When the mobile device is brough inproximity with a body part of the user, the PROXTIMESTAT flag willimmediately turn to active.

TABLE variables symbol meaning A(n) baseline estimation D(n)baseline-subtracted data R(n) raw capacitance data U(n) useful(filtered) data PROXSTAT immediate proximity status PROXTIMECONFIGselects operation generating PROXTIMESTAT (logic AND or logic OR)PROXTIMESTAT combined proximity status threshold for the immediateproximity status TIMEAVGCOND selects whether the power is determined byPROXSTAT/PROXTIMESTAT TIMEAVGCOUNT how many ‘1’ are currently present inthe FIFO buffer TIMEAVGDURATION length of the FIFO buffer TimeAvgFifoThe FIFO buffer: up to 256 past values TIMEAVGGRAN time granularity ofthe insertion in the FIFO buffer TIMEAVGINIT defines how the FIFO bufferis initialized TIMEAVGSTAT time-averaged proximity status TIMEAVGTHRESHthreshold for TIMEAVGSTAT to be set TimeGranCount counts how many timesPROXSTAT was set during the current TIMEAVGGRAN interval

Several improvements and perfectioning to the present invention arepossible. On one hand, the implementation of FIG. 2 uses a counter 280to reduce the depth of the memory buffer 250. This is advantageousbecause the regulations require long averaging times (of the order ofseveral minutes) and, if updated at full rate, the buffer 250 would needto be inordinately long. A counter is a means to implement a granularityinterval longer than the time occurring between two successive values ofthe proximity flag PROXSTAT but it is not the only possibility.

The unit 280 could be any device that converts a stream of PROXSTATvalues into an output stream with a lower rate, with a rate reductionratio TIMEAVGGRAN. Each value of the output stream could be a summary ofseveral PROXSTAT values, for example a count of the ‘1’ valuesquantized, as disclosed above, or a maximum. In the important case of aone-bit implementation, the rate compression unit 280 could beconfigured to yield ‘0’ if all the previous TIMEAVGGRAN input values are‘0’, and ‘1’ in all other cases, which is equivalent to a maximum.

Ideally, the TIMEAVGCOUNT variable 360 should be a measure of the trueaverage of the distance, or at least of the proximity signal PROXSTAT ina sliding time window that has the same size as the window prescribed inthe regulatory SAR measurements. In the USA, regulation call for awindow of a few seconds to 100 seconds, depending on the frequency of.In the rest of the world the window is 6 minutes. In the device of theinvention, the length of the integration window is determined by thedepth of the FIFO buffer 250 (D or TIMEAVGDURATION) and the granularityM introduced by counter 280, i.e. the number of PROXSTAT samplesconsidered to create one entry in the FIFO. The invention is not boundto special values of D and M but, in a typical implementation, D may beselectable between 2 and 256 positions, while the granularity M nay be anumber between 1 and 16. The scan period T_(scan) can usefully between afew tens of milliseconds to a few hundred of milliseconds. A typicalvalue of the scan period is 100 ms.

The total duration of the integration window is M·D·T_(scan). If thesize of the FIFO buffer D is large—that is, the resolution of the FIFOis fine—the granularity M can be proportionally coarser. A wider rangecould be achieved by defining M=2^(k) with k=0:7 for instance with noreduction is the relative resolution of the product ( 1/256).

Ideally, the time averaged trigger should:

-   -   1) calculate the moving sum S[n] of the PROXSTAT flag (having        values 0 or 1) over D samples    -   2) compare the moving sum to a threshold TH and set PROXTIMESTAT        for the next sample

${S\lbrack n\rbrack} = {\sum\limits_{i = 0}^{D - 1}{PROXSTA{T\left\lbrack {n - i} \right\rbrack}}}$PROXTIMESTAT[n + 1] = 1whenS[n] ≥ TH

A refinement is possible: when the PROXTIMESTAT flag is set, power isexpected to be reduced, consequently, the user is no longer exposed to ahigh power even if the device is in close proximity (PROXSTAT[n]=1);hence, a feedback can be introduced as shown below:

P[n] = PROXSTAT[n] ⋅ (1 − PROXTIMESTAT[n])${S\lbrack n\rbrack} = {\sum\limits_{i = 0}^{D - 1}{P\left\lbrack {n - 1} \right\rbrack}}$PROXTIMESTAT[n + 1] = 1whenS[n] ≥ TH

A FIFO buffer 250 is a suitable way to calculate a sliding sum. In anhardware implementation, a FIFO of D places may be implemented as ashift register SR[0:N−1] whose content is shifted at each insertion of anew sample P[n] in SR[0]. In software, there are more possibilities, forexample a structure like the hardware realization or a buffer with anindex in which the data are not moved but the oldest data areoverwritten by the newest sample. The index is incremented and wrapsaround when it reaches D.

For reasons of cost, area and complexity, it is desirable to limit thedepth D of the FIFO to a reasonable size (say, 256 samples). As one newsample P[n] is produced every scan period T_(scan), the duration coveredby the time averaging calculation above is T_(avg)=T_(scan)×D. Typicalnumbers are T_(scan)=100 ms, D=256, which leads to T_(avg)=25.6 s. Theregulations, however, allow averaging over up to 6 minutes (360 s). Totake full advantage of this duration the depth of the FIFO could beextended, but the cost would not be acceptable. Besides, the scan periodcould be much shorter than 100 ms in some cases, which would furtherincrease the needed size of the FIFO. Should be T_(scan)=2 ms, forexample, a FIFO capable of holding 6 minutes of PROXSTAT data would needto have D=360 s/2 ms=180,000 positions.

The multiplication of positions in the FIFO can be mitigated byprocessing each entry P[n] of the FIFO such that each P[n] holds an‘aggregation’ of N PROXSTAT values, each of them produced in a scanperiod T_(scan). A new P[n] is produced every M·T_(scan)

P[n]=ƒ(PROXSTAT[n,j] _(j=1) ^(M) i)·(1−PROXTIMESTAT[n])

where ƒ(PROXSTAT[n,j]_(j=1) ^(M)) is a suitable aggregation function ofM samples. In the examples disclosed so far, the aggregation resultsfrom the work of the data compression unit 280. The duration of themonitoring of PROXSTAT is now T_(avg)=M·D·T_(scan). The challenge liesin the definition of the aggregation function ƒ. Ideally, ƒ would be theaverage of all the M PROXSTAT flags, but this would require log₂(M) bitsfor each entry in the FIFO, which is still undesirable. In the examplesshow previously the function ƒ takes the maximum of the M PROXSTATflags. since the flags are binary-valued, a single high value of thePROXSTAT flag is all it takes to insert a ‘1’ value in the FIFO. This isthe most conservative approach which maximizes the radiation exposuremargin at the expense of the connectivity: by picking the maximum value,the sum 360 (see FIG. 2 ) of the FIFO content is always higher than thetrue average of the PROXSTAT in the integration window of interest. Themost liberal approach, on the other hand, would be to take the minimumof the M PROXSTAT flags, i.e. a ‘0’ value would be returned if any ofthe M flags was low. In this approach the sum of the FIFO buffer wouldalways be below the true average. The resulting process would enhanceconnectivity but it would hardly guarantee the regulatory radiationexposure level.

In a favourable embodiment, the rate compression unit 280 is configuredto yield an encoded value ƒ(PROXSTAT[n,j]_(j=1) ^(M)) such that the sumof the values stored in the FIFO buffer represents the true average ofthe PROXSTAT in the integration window of interest. This can be obtainedin several ways, one possibility being when the compression unit 280 hasthe structure of FIGS. 10 and 11 , that is that of a sigma-deltamodulator.

FIG. 10 is a generic representation of a first-order sigma-deltamodulator with an analogue input. It receives an IN value, and thedifference unit 1004 subtracts from the input value the analog value fedback from the output via the digital-to-analog converter 1024. Theresult is processed by integrator 1005 that generates an accumulatedvalue, followed by a comparator 1009 that acts as a quantizer andgenerates a digital output. The modulator operates in discrete time,thanks to the latch 1008 that is clocked by the clock signal generatedby 1026.

Sigma-delta modulators are normally followed by a decimation filter. Inthis example, the decimation filter is made by the FIFO buffer and sununit disclosed above. They are known with an analogue input, and areindeed used to provide a digital representation of the same, but theycan also function when the input is a digital value.

FIG. 11 shows a rate reducing unit with a sigma-delta modulator in lieuof the counter 280 of FIG. 2 ; we will show that this structure leads toa true time averaging. As in previous examples, the variable PROXSTAT310 at the input is combined in an ‘and’ gate 1010 with the complementof the PROXTIMESTAT variable, or of the TIMEAVGSTAT variable, comingfrom ‘not’ gate 1015. Thus, the PROXSTAT information is considered inthe modulator only when full-power transmission is allowed. WheneverPROXTIMESTAT is equal to ‘1’ the modulator is fed ‘0’ values and theuser exposure is low (same effect for low power or high distance). Theelements 1010, 1004, 1005 represented with a thick border operate atfull rate, being updated at each scan period T_(scan), for example, ateach occurrence of a new PROXSTAT value, while the element 1009, 1008,1015, represented with a dotted border, operate at a rate reduced by afactor M, or every M scan periods. The values PROXSTAT, PROXTIMESTAT maybe quantized digital values, and in this case the operation of thesumming node 1004 and integrator 1005 can be implemented by updatingrepeatedly a digital accumulator Acc[n+1]=Acc[n]−Q+PROXSTAT(disregarding the value of TIMEAVGSTAT fed back to gate 1010), where Accdenotes a digital value presented at the input of the discriminator. Thefollowing disclosure will refer to the accumulator Acc, or to theintegrator 1005 equivalently.

The sigma-delta converter reduces the rate of the input stream by agiven ratio. The processing done by this circuit can also be described,perhaps more intuitively, as a circuit configured to:

-   -   calculate the average rather than the worst case of each        (M·T_(scan)) window, then pass this average to the FIFO that        takes the sum of all these averages.    -   represent the average of the (M·T_(scan)) window and still        represent this average with a single bit. Using the delta-sigma        modulator, the fractional part of the average of each        (M·T_(scan)) window is carried forward to the next window.        To illustrate the preceding, supposing that the granularity        interval M=16 and the sequence of the PROXSTAT values has        respectively 5, 8 and 9 values high in each of three        (M·T_(scan)) periods. The first entry in the FIFO would he 0        (only 5 high flags out of a maximum value of 16) and an        accumulated value of 5 is carried forward to the next window.        The second window has 8 values set and the accumulated value is        now 5+8=13. The second value in the FIFO is then ‘1’ and a value        ‘1’ will be subtracted from the accumulated value for the next        M=16 scan periods. During that time, there will be 9 new high        PROXSTAT flat, therefore, the value accumulated at the end of        the third (M·T_(scan)) window will be 13−16+9=6, etc.

For instance, if the granularity interval is again taken as M=16 (16values of PROXSTAT to be resumed in one value pushed into the FIFObuffer) in a first cycle 16 PROXSTAT values will be summed into thenumeric accumulator 1005, unit 1002 is a M-factor rate compressor that,at the end of a granularity cycle, will let forward the accumulated sumto the comparator 1009 which will generate an output value of ‘1’ if theaccumulated sum exceed a stated threshold, which could be 8=M/2. Incontrast with the accumulator of the previous example, the accumulator1005 is not reset to zero at the end of a granularity interval but theoutput value of the sigma-delta converter is subtracted from the input,multiplied by a factor M introduced by the M rate expander 1006.Simulations have shown that the averaged proximity values generated byin this way are more reliable than that of the previous example.

FIG. 12 shows the delta-sigma modulator augmented with the FIFO buffer250 and adder 220, like in FIG. 2 . The dashed box 520 encloses theelements that operate at a reduced rate, once every (M·T_(scan))periods, while the elements 1010, 1005, 1004, represented with a thickborder, operate at full rate.

Since the threshold value applied to the comparator 1009 is M/2, thelowest value accumulated occurs when the comparator outputs a ‘1’ andthe M following PROXSTAT values are ‘0’. The accumulator is decrementedM times and its value after receiving these M PROXSTAT=0 isAcc_(min)=M/2−M=−M/2. Similarly, if the accumulated value is just belowM/2, Q=0 at the latch 1008 and the highest accumulated value occurs ifthe M following PROXSTAT values are ‘1’: Acc_(max)=M/2+M=3M/2.

To simplify, the threshold value applied to the trigger 1009 could beset to any value, and it could be zero. The choice of M/2 causes theoutput to match the true average from the first cycle. Setting it to 0would change the behaviour only after reset. The range of theaccumulator would be [−M, M].

When the FIFO buffer 250 is full the total number of PROXSTAT valuesapplied to the input of the modulator is approximated by the sum 365 (S)of the ‘1’ values in the FIFO, such that {circumflex over (N)}=S×M whereN denotes the exact number of ‘1’ values at the input of the modulatorand {circumflex over (N)} is its approximation based on the FIFOcontent. The difference between N and {circumflex over (N)} can bebounded. At the end of each (M·T_(scan)) period, a new value of theaccumulator (Acc) is calculated. Assuming that the initial state of themodulator at period 0 is Acc=Acc[0] and Q=Q[0], the value of theaccumulator at the end of period 1 is

${{Acc}\lbrack 1\rbrack} = {{\sum\limits_{i = 1}^{M}{PROXSTA{T\lbrack i\rbrack}}} - {M \cdot {Q\lbrack 0\rbrack}} + {{Acc}\lbrack 0\rbrack}}$

which can be extended to the end of the D^(th) (M·T_(scan)) period

${Ac{c\lbrack D\rbrack}} = {{\sum\limits_{i = 1}^{M}{{PROXSTA}{T\left\lbrack {i + {\left( {D - 1} \right)M}} \right\rbrack}}} - {M \cdot {Q\left\lbrack {D - 1} \right\rbrack}} + {Ac{c\left\lbrack {D - 1} \right\rbrack}}}$

Summing terms for Acc[1:D] we get:

${\sum\limits_{i = 1}^{D}{{Acc}\lbrack i\rbrack}} = {{\sum\limits_{i = 1}^{M + D}{PROXSTA{T\lbrack i\rbrack}}} - {\sum\limits_{i = 1}^{D}{M \cdot {Q\left\lbrack {D - 1} \right\rbrack}}} + {\sum\limits_{i = 1}^{D}{{Acc}\left\lbrack {i - 1} \right\rbrack}}}$

simplifying, reordering, and remembering that Σ_(i)PROXSTAT[i]=N

$N = {{M \cdot {\sum\limits_{i = 1}^{n}{Q\left\lbrack {i - 1} \right\rbrack}}} + {{Acc}\lbrack D\rbrack} - {{Acc}\lbrack 0\rbrack}}$

generalizing to any scan period of index n

${N\lbrack n\rbrack} = {{M \cdot {\sum\limits_{i = {n + 1 - D}}^{D}{Q\left\lbrack {i - 1} \right\rbrack}}} + {{Acc}\lbrack n\rbrack} - {{Acc}\left\lbrack {n - D} \right\rbrack}}$

Σ_(i=n+1-D) ^(n)Q[i−1] is the sum of the FIFO (S) before the shift inQ[n]; hence,

N[n]=M·S[n−1]+Acc[n]−Acc[n−D]

N[n]={circumflex over (N)}[N−1]+Acc[n]−Acc[n−D]

In all cases, to meet the regulatory SAR limitations the highestpossible value of N must be considered based only on its approximation{circumflex over (N)}. The smallest and largest possible values ofAcc[n−D] are −M/2 and 3M/2; hence, a blind scheme would require that theFIFO sum S be augmented by 3M/2−(−M/2))/M=2 before comparison toguarantee that {circumflex over (N)}+2≥N and that the regulation bealways satisfied. Alternatively, the threshold TH could be lowered by 2.

The scheme can be improved since Acc[n] is known and is held in thecurrent accumulator. Comparing that value to M/2 allows to decidewhether to add only 1 (when Acc[n]<M/2), 2 otherwise. This condition isalready known at the end of period [n] and is contained in Q[n]. Theupper bound on S (S_(UB)) is:

S _(UB) [n]=S[n−1]+Q[n]+1

Finally, the threshold for the accumulator quantization can be set toany arbitrary value. The easiest for implementation is zero. The reasonfor choosing M/2 initially is that this matches the behaviour of theexact/match case and corresponds to the definition of averageconsidering that the output signal is 0/1, i.e., has a bias of 0.5.Changing from M/2 to 0 will make the first TIMEAVGSTAT decision (whenthe FIFO is filled for the first time) pessimistic by ½, and this termwill disappear in subsequent decisions.

FIG. 13 shows an improved version of the processor in which theaccumulator threshold in the modulator is set to zero and the FIFO sumis bound below a maximum. As before, the dashed box 520 indicates theelements that are clocked at the reduced rate.

The method can be represented by the flowchart of FIG. 14 . Step 402 isan initialization procedure that is carried out only at the beginning ofthe processing, and may include, among others, initializing the FIFO,resetting the output Q=0 of the latch 1008, resetting of the accumulator(the output of the integrator 1005 after the rate downconversion 1002,and setting to zero various counters. Successive steps will refer to acounter m that is incremented repeatedly from 0 to M and is used todetermine the position inside a (M·T_(scan)) period. This counter mayalso be reset to zero in initialization 302.

In step 405, the method wait until a new PROXSTAT value is available. Inprinciple, the method is executed and looped back to step 305 in eachT_(scan) interval.

Step 420 represents the update of the accumulator value Acc or,equivalently, the actions of integrator 1005 and adding node 1004

Acc[n+1]=Acc[n]−Q+PROXSTAT∧TIMEAVGSTAT

where ∧ denotes the AND operation. in Step 422 the counter m mentionedpreviously is incremented.

Test 455 is used to determine the end of a (M·T_(scan)) period. Theaccumulator is updated M times until the control is passed to test 440that stands for the action of the comparator 1009 and latch 1008. Afterthat, the sum of the FIFO is computed (step 460), a new value of Q ispushed in the FIFO (step 470), comparator 260 compares the summed valuewith the threshold (step 475) and the TIMEAVGSTAT value is setaccordingly (steps 480, 481), the counter m is reset to zero, the valueof PROXTIMESTAT is updated (step 490) and the cycle restarts.

FIG. 15 shows the action of the proximity sensor of the invention in asmartphone. The device is tested by approaching a “phantom”, simulatinga body part, to the smartphone, and measuring the electromagneticradiation absorbed in the phantom. During the test, the distance dbetween the phantom and the phone decreases initially from 50 mm to 0 mm(zone A of the plot), the phantom is then kept in contact with the phonefor a certain time (zone B) and is finally moved away until the distanceis again 50 mm (zone C). The dashed line ‘PS off’ represents the dosewhen the phone has no mitigation mechanism: the radio power is notadapted at all. The dotted line ‘TA off’ is the response obtained whenthe RF power is controlled by an immediate proximity flag like PROXSTAT.The power decreases abruptly when the distance between the approachingphantom and the phone reaches a certain threshold value, and is restoredto the initial value when, the phantom is at the same distance in itsretreat motion.

The solid line TA-PS is the result of the time-averaged proximity flagof the invention. A little while after the approaching phantom crossesthe threshold, the RF power is decreased, but it is momentarily raisedto the full value in a cyclical fashion, even while the phantom is incontact. In this way the time-integrated dose is reduced, but theconnectivity is not as degraded as in the previous case.

In another embodiment, not represented in the drawing, the proximitydetector of the invention may dispense with the averaging FIFO and togenerate a cyclic proximity signal that causes the RF power to bereduced cyclically when the portable device is near the user. In thismanner, the RF power alternates between a high value and a Low valuefollowing a simple periodic rule with a determined duty cycle. In thismanner, the SAR is reduced with less impact on the connectivity. Whenthe immediate proximity flag indicates that the wireless device is nearthe user, the radio power is periodically set to a lower value, and thenback to the higher normal value. Simplicity is a distinct advantage ofthis variant that is very suitable to low-cost devices.

In the above examples, the variable that is applied to the input 310 isan immediate proximity status PROXSTAT that is raised as soon as theproximity sensor senses that a given capacitive electrode is near to agiven RF antenna, but the proximity sensor may in fact be capable todetermine proximity with respect to several antennas and capacitiveinput electrodes of the portable device. In an advantageous variant, theproximity sensor is configured to build a logic combination of proximitysignals coming from several antennas and/or input electrodes and presentthis combined signal at the input 310 for time-averaging. For example,the signal may be a sum or a logical OR or any suitable logic functionof the proximity signals from multiple antennas/electrodes.

The proximity sensor may be configured to discriminate high-permittivitybodies and low-permittivity bodies, the former ones being indicativethat a part of a user's body is near. Other proximity sensors have meansto discriminate a legitimate proximity from contamination, for exampledew or water on the portable device. In proximity sensor with thesecapabilities, the signal given to the input of the time-averaging unit310 may be a qualified proximity signal BODYSTAT that is raised forbody-like proximity and not otherwise. Several devices are possible tobuild a qualified proximity signal that is raised when a body part of auser is near but is considerably less sensitive or essentiallyinsensitive to inanimate bodies or contaminations, and all are includedin the present invention. Such qualified proximity signals take mostoften binary values but they can be multi-level as well.

As mentioned above, an important realization of the invention operateson a one-bit binary signal, but this is not the only case, and in factthe invention can be adapted to operate on non-binary signal, insofar asthey can be represented by digital word of a suitable number of bits.For example, the proximity sensor may be configured to generate amulti-level immediate proximity signal where different values correspondto multiple distances. In this case, the distance level can be encodedin binary form, for example in a two-bits word capable of representingfour distance levels from 0 (no proximity) to 3 (nearest), which ispresented at the input 310. Naturally, the rate compressor 280, FIFObuffer 250 and sum function 220 will have the suitable bit width.

In another variant of the invention, the value presented at the input310 for time-averaging may be, rather than the binary-valued PROXSTAT,the digital value D(n) representing the self-capacitance of theantennas/electrodes as determined by the ADC and after filtering andsubtraction of the baseline.

The present invention implements a windowed average using a memorybuffer that has as many taps as required by the duration of theaveraging interval (with M or TIMEAVGRAN) as scaling factor. Whileprecise, this requires a sizable memory. The averaging unit may include,rather than a sliding-window averager with a FIFO buffer, a generaldigital low-pass filter that can be implemented more compactly, forexample a recursive low-pass digital filter (an IIR filter).

The present disclosure also includes the appendixes “time averaging 2.0”and “Time averaged proximity sensor” appended hereto.

REFERENCE SYMBOLS IN THE FIGURES

-   -   Ld decoupling inductance    -   Cx tactile capacitance    -   Cd decoupling capacitance    -   20 electrode    -   23 analogue processor    -   25 A/D converter    -   30 filter    -   40 difference    -   50 discriminator    -   60 baseline estimator    -   70 threshold    -   90 RF receiver/transmitter    -   105 generation of a new PROXSTAT value    -   120 counting    -   122 end of granularity interval    -   130 push into shift register    -   135 update TIMEAVGCOUNT    -   140 comparison with TIMEAVGTHRESH    -   150 set time-averaged proximity flag    -   160 reset time-averaged proximity flag    -   170 logic operation    -   220 sum    -   230 baseline estimation    -   240 drift-corrected samples    -   250 FIFO buffer    -   260 comparator    -   270 logic operation    -   271 logic gate    -   272 logic gate    -   273 logic gate    -   280 counter or rate compression unit    -   310 PROXSTAT variable    -   320 TIMEAVGTHRESH variable    -   330 TIMEAVGSTAT variable    -   340 PROXTIMECONFIG variable    -   350 PROXTIMESTAT variable    -   360 TimeAvgCount variable    -   365 True average    -   370 serial input of the FIFO buffer    -   402 initialization    -   405 generation of a new PROXSTAT value    -   420 update accumulator/integrate    -   422 increment counter    -   435 test counter    -   440 test accumulator    -   451 set Q    -   450 reset Q    -   460 sum FIFO    -   470 push new value of Q into FIFO    -   475 test sum    -   480 reset TIMEAVGSTAT    -   481 set TIMEAVGSTAT    -   490 set PROXTIMESTAT, logic operation    -   520 downscaled rate    -   1002 rate downconversion    -   1004 difference    -   1005 integration    -   1006 rate upconversion    -   1007 delay    -   1008 latch    -   1009 trigger    -   1010 and gate    -   1015 not gate, inverter    -   1024 digital/analog converter    -   1026 clock

1. A proximity sensor for a portable wireless connected device, thesensor being arranged to determine whether a part of a user's body isnear the portable connected wireless device, the sensor comprising aprocessing circuit generating an immediate proximity status flag thatbecomes active when a part of a user's body is close to the proximitysensor, characterized by an averaging unit configured to generate arunning average of the immediate proximity status flag in apredetermined time window, and by a decision unit generating atime-averaged proximity status flag based on an averaged or accumulatedvalue of the immediate proximity status flag in the time window.
 2. Theproximity sensor of claim 1, the decision unit being configured toswitch the time-averaged proximity status flag to an active state whenthe running average exceeds a predetermined threshold.
 3. The proximitysensor of claim 1, the decision unit being configured to switchtemporarily and repeatedly the time-averaged proximity status flag to aninactive state when the immediate proximity status flag is active. 4.The proximity sensor of claim 1, the averaging unit including asigma/delta modulator configured as a rate compression unit and a FIFObuffer that is periodically supplied with values of the rate compressionunit.
 5. The proximity sensor of claim 1 wherein the immediate proximitystatus flag is any one of: a binary proximity flag indicating that thedetector is near a conducting body, a qualified binary proximity flagindicating that the detector is near a body part of a user but with alower sensitivity for inanimate objects and/or contaminations, amulti-level proximity flag encoding distance to the conducting body, adigital value issued from the conversion of a self-capacitance of asense electrode and/or a self-capacitance of a radio antenna, a combinedproximity flag produced by a logic function of individual proximityflags each derived from the self-capacitance of a distinct senseelectrode or radio antenna.
 6. The proximity sensor of claim 4, the FIFObuffer having a selectable length.
 7. The proximity sensor of claim 1,including a logic circuit configured to inhibit the transmission offurther values of the immediate proximity status flag to the averagingunit if the time-averaged proximity status flag is active.
 8. Theproximity sensor of claim 1 in combination with a portable connectedwireless device that includes a radio transmitter, the proximity sensorbeing operatively arranged to reduce a power of the radio transmitterbased on the value of the combined proximity status flag or of theimmediate proximity status flag or of the time-averaged proximity statusflag.
 9. The proximity sensor of claim 8, the sensor being a capacitivesensor arranged to determine whether a user is in proximity to theportable connected wireless device based on a capacitance seen by asense electrode.
 10. The proximity sensor of claim 9, the senseelectrode being also an antenna for emitting radio waves.
 11. A methodof reducing a dose of integrated SAR in a user of a wireless devicecomprising, in a temporal cycle:
 1. obtain an immediate proximity flagindicating whether the wireless portable device is momentarily near theuser
 2. computing a running average value of the proximity flag based onpresent and past values of the proximity flag and determine atime-averaged proximity flag based thereon
 3. reduce a power of a radioemission of the wireless device when the time-averaged proximity flag isasserted
 12. The method of claim 11, including resetting the power tothe initial value momentarily in periods when the immediate proximityflag is asserted.
 13. The method of claim 11, including reducing therate of the immediate proximity flags before the computation of theaverage value to a second rate lower than a first rate of the temporalcycle by providing the immediate proximity flag to a sigma/deltamodulator.
 14. The method of claim 11, wherein the computing of anaverage value comprises pushing cyclically the value of the immediateproximity flag into a FIFO buffer and computing a sliding-window averagefrom the sum of all the values in the FIFO buffer.